1. Field of the Invention
Pyrotechnic initiators are needed in gas generators for passive safety systems such as airbags and belt tensioners in motor vehicles.
Such gas generators work with initiators, by means of which a propellant charge can be ignited pyroelectrically.
Such pyrotechnical devices guarantee a reliable ignition of the propellant charge and function flawlessly over many years. For this assignment, such devices must assure a function over decades.
This condition requires that a flawless function is guaranteed permanently and that the igniter and also the propellant charge are not changed during the service time. It must be assured that, for example, no moisture can penetrate into the gas generator. For this assurance, the propellant charge and the igniter must be hermetically encapsulated. At the same time, it must be assured that the gases are released in the correct direction upon the ignition of the propellant charge.
2. Description of the Related Art
Among electrically ignited devices, ignition elements or micro gas generators, two basic types now exist in order to anchor the two current-conducting contacts of a pyroelectric structural element in the base or pole carrier of such an igniter. For this anchoring, a pole carrier is needed in which one or more contact pins are embedded insulated from one another in gastight manner and can be guided from outside into the ignition device via the electrical pulses.
Essentially two methods, which respectively have specific advantages and disadvantages, have been adopted in practice.
1. Glass-to-Metal Seal
A corresponding glass-to-metal seal is characterized in that at least one of two or more contact pins is fused in glass and therefore fixed. The glass base in turn is anchored in a metal ring, which permits a gastight joint—for example by laser welding—for the remaining construction of the igniter and at the same time can form one pole of the electrical ignition device if necessary.
The construction of igniters and other pyrotechnical elements with glass-to-metal seal provides for making the container with the actual pyrotechnical ignition mass of metal and, after pressing onto the metal ring of the glass-to-metal seal, welding it with this metal ring.
After an ignition bridge has been installed on the contact pins and the actual pyrotechnical ignition mass has been installed, this already functional igniter can also be equipped with a plastic cap and if necessary overmolded by means of plastic, in such a way that a standardized interface with the system surrounding it is formed.
Such a construction can be inferred, for example from the publication DE 696 03 082 T2, which has as subject matter a pyrotechnical electrical igniter and a safety device containing such an igniter for motor vehicles. This ignition base formed as an electropyrotechnical switch has two electrical terminal pins which, embedded in a glass or ceramic body, are connected to one another by an electrical ignition bridge. This ignition bridge is formed as an electrical microcomponent.
A second unit forms the pyrotechnical part of the initiator or ignition element. This part consists of a metallic cup, which has an insulating insert, in which a pyrotechnical composition is housed.
The two units are pressed together with one another and hermetically laser-welded together with one another by means of the metal ring embracing the glass or ceramic base.
A space, which is filled with a helium-saturated material, is formed in the interior of the metallic cup, and so possible leaks of the initiator or ignition element can be detected.
The processes for this construction are very complex, in part because many expensive components are needed and in addition the number of manufacturing steps is high. In particular, the welding of the metal cup with a metal ring proves to be not unproblematic, because a test for absolute leaktightness of such an igniter is possible only conditionally and only with considerable complexity.
In order to be able to fix a bridge wire in a glass-to-metal sealed igniter to the contact pins in such a way that it then rests flat on the glass between the two contact pins, the surface must be reworked by grinding. An advantage of the glass, however, is that it has a much higher melting point than the bridge wire and thus no kind of cavities or other changes between glass, bridge wire and the ignition mixture can occur during the testing or ignition by means of a current pulse due to the heat development that then occurs.
Another configuration of the ignition base of the type under discussion follows from U.S. Pat. No. 5,988,069. The ignition base described in that publication also has two contact pins, insulated from one another, which are fixed in hermetically sealed manner in the ceramic base by means of glass seal. On its upper side, the ceramic base is combined with a metal base, through which the one contact pin is guided in insulated manner, whereas the other contact pin is securely connected in electrically conductive manner with the metal base. The metal base and the contact pin insulated from it are electrically connected with one another via a bridge wire. The bridge wire is in contact with an ignition charge and this ignition charge with a propellant charge. This arrangement is enclosed with a metal cup, which is hermetically laser-welded with the metal base. The metal cup is finally also covered by a plastic cap. Below, a further metal base encloses the ignition base, with which it is pressed with use of a seal.
This construction has the combination of a large number of different components with a corresponding number of working steps for its assembly, which is costly and from the viewpoint of production technology is complex.
In U.S. Pat. No. 5,230,287, an ignition element for a micro gas generator is described in which the ignition element is constructed in the form of a semiconductor bridge. The semiconductor bridge is housed together with a propellant batch in the form of a pyrotechnical compound in a metal container, which is joined in hermetically sealed manner with an electrically non-conducting base by means of an adhesive compound. Electrical contact pins, which extend up to the semiconductor bridge and via which an ignition of the propellant batch can be achieved from pyrotechnical material, are fixed in the base. At least one of the contact pins is then completely electrically insulated from the metal container, whereas the other is connected with the ground of the device. Both contact pins are retained in metal penetrations, in which they are fixed by means of glassy insulating seals. The contact pins with their metal guides are additionally fixed in a central body and in the container with the pyrotechnical material by means of a semi-rigid adhesive, which represents a second hermetic seal. This construction is also complex and therefore costly.
2. Plastic-to-Metal Seal
The plastic-to-metal seal represents a further possibility for the formation of an ignition base of the type under discussion.
In this construction, the contact pins are anchored directly in a plastic base, which simultaneously forms the standardized interface to the system surrounding it. A possible and practiced manufacturing method is then to insert the contact pins into an injection-molding die and then to overmold them. This type of sealing has the advantage that only one or more contact pins and the plastic are needed in order to manufacture the component. Thus few and in addition inexpensive supply materials are necessary for a reduced number of process steps.
Disadvantageously, it turns out that the coefficients of expansion between contact pin and plastic lie very far apart from one another and that the plastic can melt above 300° C. A bridge wire, which can reach between 600° C. and 1,000° C. during ignition, in this case melts a part of the base, so that bridge wire and ignition mixture in this region are no longer joined to one another in the original manufacturing condition. Furthermore it is disadvantageous that, according to specifications usual today, round contact pins are required, which during insertion into an injection-molding die can be fixed only in longitudinal direction. Thus the contact pin can be turned inside the die; if a bending or crimping of the contact pin or pins is then still necessary, the position of the contact pins can no longer be correctly controlled for fixing the bridge wire, so that an absolute reproducibility of the required arrangement is not given. As a solution to this problem, as a rule the approach is taken that the contact pins are lengthened in such a way that they can be fixed at both ends in the die, which entails a projecting length.
Because the surface for the bridge wire must be very clean and even and should not have any raised sites, grinding of the contact pins (as also for the glass-to-metal seal) must consequently be carried out subsequently at this place. Once again metal and plastic behave very differently during this grinding, so that it cannot always be guaranteed that a clean and smooth surface is then achieved.